AJP - Regulatory, Integrative and Comparative Physiology, Vol 244, Issue 5 709-R717, Copyright © 1983 by American Physiological Society
Mitochondrial ammoniagenesis in liver of the channel catfish Ictalurus punctatus
J. W. Campbell, P. L. Aster and J. E. Vorhaben
Rates of ammonia formation from six amino acids by hepatocytes and liver
mitochondria were compared with the rate of ammonia excretion by individual
fish. Glutamine and asparagine are the most ammoniagenic substrates for
hepatocytes and glutamine and glutamate, for isolated liver mitochondria.
The main site of ammonia release is the mitochondrion. Ammonia is formed
mainly via deamidation or transdeamination. Relatively small amounts of
aspartate are formed from glutamate and glutamine by the fish liver
mitochondria. This apparent "uncoupling" of glutamate transamination may
represent an adaptation of carnivorous fish to the utilization of amino
acids as a major energy source. More than 90% of the ammonia formed from
glutamate and glutamine by isolated mitochondria are elaborated into the
medium. Exiting ammonia is accompanied by a proton, but the source of this
proton appears not to be the electrogenic proton pump or the carboxylate of
exiting glutamic acid.
